Heat the oven to 425°F. Toss the sweet potato cubes and onion wedges with 2 tablespoons of the olive oil, 1/2 teaspoon of the salt, and the pepper.
Spread them across a large sheet pan in a single layer with space between the pieces. Roast for 15 minutes.
Meanwhile, toss the sausage rounds and apple wedges with the remaining tablespoon of oil, the remaining salt, the thyme, and the smoked paprika.
Pull the pan out, push the sweet potatoes toward the center, and add the sausage and apples around them. Toss everything together on the pan.
Return to the oven for 18 to 20 minutes, stirring once at the halfway point, until the sweet potatoes are tender and caramelized and the sausage edges are browned.
Drizzle the apple cider vinegar over the hot pan and toss — it lifts the whole dish and balances the sweetness.
Serve straight from the pan, with extra thyme if you like.